Autoimmune diseases like rheumatoid arthritis (RA) develop when the immune response targets self- antigens, leading to inflammation and tissue destruction. There is now considerable evidence that recognition of self nucleic acids through toll-like receptors (TLRs) can contribute significantly to sterile inflammation and autoimmunity, with the clearest example being the role played by TLR9 and TLR7 in the pathogenesis of systemic lupus erythematosus (SLE). Similarly, TLR8, a potent stimulator of inflammatory cytokines such as IL6 and TNF-?, has a strong association with RA and other autoimmune indications. However, the lack of useful animal models, a consequence of the different ligand specificity of human TLR8 and its rodent orthologs, has proven to be a major limitation in the study of TLR8 function and the identification of possible therapeutics. We have developed new tools, including in vivo animal models that have allowed us to better understand the biology of TLR8 in autoimmunity and specifically for a role of TLR8 signaling in RA. We propose to use these tools to identify a lead small molecule antagonist for developing a therapeutic for TLR8- mediated autoimmunity. The key objective of this Phase II proposal is to identify lead small-molecule antagonists of human TLR8 from a set of candidate hits that we have identified through high-throughput screening with funding from a Phase I SBIR. The principal activities will include: 1). Advancing hits and identifying lead compounds with specific and improved huTLR8 antagonist activity through a process of iterative computer-aided drug design with structure-based modeling and cell-based activity; 2). Determining the activity of the lead candidates in TLR8-dependent RA mouse model; 3). Conducting preliminary toxicology of lead compounds to support IND-enabling studies If successful, we anticipate obtaining a small number of lead small molecule candidates that inhibit TLR8 and have sufficient attributes (e.g., potency) to move into IND enabling studies.